SAS can be defined by a significant recurrence of apneas or hypopneas associated with clinical symptoms. An “apnea” or “respiratory pause” is a temporary stop of the respiratory function of a duration longer than 10 seconds that occurs during a phase of sleep of the patient. A “hypopnea” is defined as a significant decrease of the minute ventilation, for example, more than 50%, compared to an average of prior minute ventilation measures, but without a stoppage of the respiratory function. The minute ventilation is the product of the amplitude by the frequency of the successive respiratory cycles.
Various techniques to detect these sleep respiratory disorders are known. Among these techniques are those that implement various types of sensors, in particular a sensor of minute ventilation, a parameter having a physiological preponderance that is obtained by an intrathoracic impedance measurement operated by an implanted device.
The measurement of the minute ventilation is carried out by an injection of pulses of a constant current having an amplitude of a few hundred microamperes at a frequency of a few hertz. The current injection can take place, for example, between two electrodes laid out in the rib cage, or between the case of the implanted medical device and an electrode, e.g., a stimulation electrode if the implanted device is a cardiac pacemaker. The measured impedance variations reproduce the variations of the thoracic volume, with peaks of impedance at the time of the inspiration when the lungs are filled with air, and a decreasing impedance at the time of the expiratory phase.
Other techniques have been proposed, based on the measure of a parameter having a physiological preponderance such as the oxygen saturation in blood. In this regard, a prolonged reduction of respiratory activity produces a correlative desaturation of oxygen. This condition can be detected by an adapted sensor, for example, placed at the end of a catheter or a probe of a cardiac pacemaker.
It was noted in practice, however, at the time of clinical studies, that a system implementing a respiratory activity sensor, recording the variations of the pulmonary volume at the thoracic level can in certain circumstances be deluded during the sleep for various reasons:    1. internal physiological reasons, for example, a body position or a movement inducing a modification of volumes and/or position of the concerned organs,    2. clinical reasons, for example, because of breathing that is exclusively abdominal in origin, or    3. external reasons, for example, because of momentary electromagnetic interference.
In these particular situations, this device detects wrongly the occurrence of an apnea or of a hypopnea, whereas the breathing is actually normal but was not diagnosed as such by the device. In the same way, an apnea or a hypopnea can cause a micro-waking up of the patient without affecting to a significant degree the blood oxygen saturation.